Effect of CaO-doped in NiMn₂O₄–LaMnO₃ composite ceramics on microstructure and electrical properties

Abstract

The composite ceramics (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 (0 ≤ x ≤ 0.3) consisting of spinel-structured NiMn₂O₄ and perovskite-structured CaO-doped LaMnO₃ were prepared by classical solid state reaction. The X-ray diffraction (XRD) patterns have shown that the major phases presented in the sintered samples are NiMn₂O₄ compounds with a spinel structure, La_1−x Ca _x MnO₃ with a perovskite structure. The Scanning Electron Microscope (SEM) pictures have exhibited that the grain size of the composite ceramics decreases from ca. 6.5 to 2.0 μm as the mole fraction of CaO increases from 0 to 0.3. The ρ _25 °C and B _25/50 constants of the composite samples are in the range of 0.234–8.61 Ω cm and 2,600–2,962 K, respectively. In particular, CaO-doped leads to a decrease in the resistance drift of the (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 composite NTC (negative temperature coefficient) ceramics after aging test. This indicates that the CaO-doped (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 NTC ceramics display high electrical stability in comparison with the Ca-free (NiMn₂O₄)_0.50(LaMnO₃)_0.50 ceramics. The X-ray photoelectron spectroscopy (XPS) analysis verifies that the valence states of the manganese ions have a highly mixed state of Mn²⁺, Mn³⁺ and Mn⁴⁺ at B site. And the electrical conduction of the composite ceramics can be elaborated by the ions migration mechanism.

1 Introduction

The NTC thermistors (TRs) based on mixed transition-metal and rare earth metal manganite electroceramics have been widely used for temperature measurement, control, compensation and suppression of inrush current, due to their high stability and low cost [1, 2]. In practice, the NTC thermistors are generally characterized by two parameters: B, the thermal constant which indicates a sensitivity to temperature excursions (B = Ea/k, unit in Kelvin, in which Ea is the electronic activation energy and k is the Boltzmann constant) and ρ _25 °C, the resistivity at 25 °C. By compounding spinel and perovskite structure oxides, varying their relative proportions, doping with Alkalierdelement and controlling the processing conditions, a right combination of the resistivity and B value can be obtained [3].

The popular NTC materials are Ni- and Mn- based spinel oxides of general formula AB₂O₄, such as Mn–Ni–Co–O, Ni–Cu–Mn–O, Fe–Mn–Ni–O, Mn–Ni–Co–Zn–O [4–7]. For those spinel oxides, a high B value generally couples with high electrical resistivity and vice versa. However, some applications such as suppressing the inrush current require NTC materials with a low electrical resistivity (≤10 Ω cm) while possessing a large B (≥2,600 K) and sufficient stability. Therefore, there is a need for the development of new materials that have good electrical characteristics in suppressing the inrush current. Traditional methods to design a new material are composite and doping. Zhao et al. [8] have studied the composites made of spinel-structured (Ni, Mn)₃O₄ and perovskite-structured La(Mn, Ni)O₃, and found that the electrical resistivity of the composite decreased depending on the amount of the perovskite phase. Luo et al. [9] have prepared a composite ceramics material with high electrical resistivity and low B by introducing yttrium-stabilized zirconia as the second phase into Fe_0.5Cu_0.2Ni_0.66Mn_1.64O₄ spinel matrix, the former is of high electrical resistivity and the latter of low electrical resistivity and low B. In principle, the material with low electrical resistivity and high B may be obtained through introducing a second phase with low electrical resistivity into a spinel oxide of high B. What’s more, the composite can make full use of favorable condition and promote complementarity in each component due to the adjustability of its multiplicity and binding form [10], and the outstanding and attractive properties that the single component couldn’t achieve may be obtained by complex method. Therefore, we propose an alternative approach by introducing a highly conductive component doped by a certain element into the spinel oxide. Perovskite structure LaMnO₃ is a good electronic conductor with the electrical resistivity of <than 1 Ω cm at 25 °C [11]. The Ca ion has a stable valence state bivalence and the Ca²⁺ has a approximate ionic radii with La³⁺(the radius of Ca²⁺, La³⁺ are 0.1000 and 0.1032 nm, respectively). Proper substitution of La for Ca may also decrease the resistivity of LaMnO₃ [12]. In the present work, the effects of CaO-doped in NiMn₂O₄–LaMnO₃ composite ceramics on microstructure and electrical properties are investigated.

2 Experimental procedure

The NiMn₂O₄ and La_1−x Ca _x MnO₃ (0 ≤ x ≤ 0.3) powder was synthesized using the conventional solid-state reaction method. The raw materials were analytical reagent La₂O₃, CaO, MnO₂ and Ni₂O₃. With appropriate amounts of those oxides, NiMn₂O₄ and La_1−x Ca _x MnO₃ powder was ball-milled at room temperature for 8 h separately in agate containers using agate balls as the milling medium and deionized water as a dispersant. The two kinds of ball milled slurry were then dried at 80 °C and calcined for 2 h at 1,000 °C respectively. The powder was ball-milled for 8 h again to break agglomerates.

With a mass ratio of the as-prepared NiMn₂O₄ powder to La_1−x Ca _x MnO₃ powder of 1:1, the mixture was grinded and then sieved. Disk-shaped compacts with a diameter of 10 mm and a thickness of 2 mm were formed by uniaxial pressing the powder at 60 MPa followed by isostatic cool pressing at 350 MPa. These green compacts were sintered at 1,300 °C for 4 h under air atmosphere.

The phase composition of the composite was determined by X-ray diffraction (XRD: BRUKER D8-ADVANCE, Germany) with Cu Kα radiation (λ = 1.5406 Å). The microstructure and element compositions of sintered samples were observed by the Scanning Electron Microscope (SEM; LEO1430VP, Germany) in combination with energy dispersive spectroscopy (EDS; OXFORD INCA200, England). The X-ray photoelectron spectroscopy (XPS: Kratos Amicus, England) analysis was performed on the compacts in an ultrahigh vacuum and the spectra were recorded using an Mg Kα X-ray source (hν = 1,256.6 eV). For electrical properties measurements, two opposite sides of the sintered compacts were coated with silver-palladium conductive paste, heated at 835 °C for metallization. The resistances of the composite were measured with disk-shaped samples using the Agilent 34401A digital multimeter. To ensure the temperature deviation from the designated value to be <±0.05 °C, samples were immersed in silicon oil. The thermal constant B was calculated according to the formula B = 3,853.89·ln (R ₂₅ /R ₅₀), in which R ₂₅ and R ₅₀ are the resistances at 25 and 50 °C, respectively. To obtain aging properties, these samples were aged by heat treatment at 125 °C in air for 500 h, and the aging is characterized by ΔR/R ₀ = (R − R ₀)/R ₀, in which R ₀ and R are the resistivity at 25 °C before and after the heat treatment.

3 Results and discussions

3.1 Phase composition and microstructure

Figure 1 shows the XRD patterns of NiMn₂O₄ (NiMn₂O₄)_0.50 (La_1−x Ca _x MnO₃)_0.50(0 ≤ x ≤ 0.3) and La_0.9Ca_0.1MnO₃ samples sintered at 1,300 °C. It can be seen that the composite ceramics are consisted of cubic structure spinel phase and orthorhombic structure perovskite phase. Before compositing, there is a little amount of monoclinic structure rock-salt NiO phase separated out from the spinel phase NiMn₂O₄ matrix and then disappeared in composites. It may be that introducing LaMnO₃ doped by CaO into NiMn₂O₄ retarded the separating process of rock-salt NiO phase. In addition, the substitution of La for Ca doesn’t change the perovskite structure of LaMnO₃. Compared to the single phase without compounding, the interplanar spacing of (111) in the spinel phase increases from 4.8561 to 4.8704 Å, while the interplanar spacing of (101) in the perovskite phase decreases from 3.8914 to 3.8824 Å in (NiMn₂O₄)_0.50 (La_0.9Ca_0.1MnO₃)_0.50 sample.

Fig. 1

The XRD patterns of NiMn₂O₄, composites of (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 (x = 0, 0.1, 0.2, 0.3) and La_0.9Ca_0.1MnO₃

The observed changes of the interplanar spacing can be explained in terms of valence and size of the cations occupying the octahedral sites. The Mn-rich spinel induced by the separation of the NiO is known to possess a cationic distribution [Mn²⁺]$\left[ {{\text{Ni}}_{\alpha }^{2 + } {\text{Mn}}_{ 2- 2\alpha }^{3 + } {\text{Mn}}_{\alpha }^{4 + } } \right]$O₄ [13]. According to this result, when a Ni²⁺ ion is replaced by a Mn³⁺ ion, to maintain the electrical neutrality of the lattice, a Mn⁴⁺ ion has to convert to Mn³⁺. In other words, annihilation of one Ni²⁺ ion is accompanied by the annihilation of one Mn⁴⁺ ion and generation of two Mn³⁺ ions. Accordingly, the sum of these two Mn³⁺ ions in size is larger than that of one Ni²⁺ ion and one Mn⁴⁺ ion based on Table 1 [14], and it explains the larger interplanar spacing observed from the Mn-rich spinel phase. As to the perovskite phase, both the migration of Ni²⁺ ions and the substitution of Ca²⁺ for La³⁺ make the change of ions valence state toward the opposite direction. The Mn³⁺ at B site in the perovskite structure is replaced by Ni²⁺ and this process leads to annihilation of two Mn³⁺ ions accompanying by generation of one Ni³⁺ ion and one Mn⁴⁺ ion which induces decrease of the interplanar spacing. Moreover, the radius of Ca²⁺ is smaller than La³⁺ and then the substitution of Ca²⁺ for La³⁺ also results in the decrease in the interplanar spacing of the perovskite phase.

Table 1 The effective ionic radii of different ions with different coordination numbers

Figure 2 shows the SEM images of the composite samples (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 sintered at 1,300 °C. From these SEM figures, it is immediately seen that the average grain size decreases from 6.5 to 2.0 μm with x increasing from 0 to 0.3, crystalline grains become integrated and uniform and the grain boundary turns more distinguishable. Obviously, the presence of the perovskite phase doped by CaO suppresses the grain growth and produces great influence on the microstructure of the composites. It is known that La can scatter electrons much more effectively than Mn, Ni and Ca, for the atomic number of the former is much larger than those of the latter. A reasonable conclusion can be drawn that the brighter area corresponds to the perovskite phase, while the darker area the spinel phase which is verified by the EDS spectra in Fig. 3. Based on the SEM/EDS results, Table 2 shows the values of weight and atomic percentages obtained from different spots in Fig. 2a4. The EDS spectra of spot A refers to the brighter area where is La-rich and Ni-poor area, while the EDS spectra of spot B referring to darker area is the contrary. From Table 2, it also can be seen that there is no pure La_1−x Ca _x MnO₃ or NiMn₂O₄ existed in the sintered body, which may indicate that the perovskite phase and the spinel phase formed solid solution during sintering process and ions migration occurred between two phases.

Fig. 2

SEM images of (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 with x = 0 La_1−x Ca _x MnO₃ (a1), 0.1(a2), 0.2(a3), 0.3(a4) sintered at 1,300 °C

Fig. 3

Representative EDS spectra of different spots in Fig. 2a4

Table 2 Values of weight and atomic percentages obtained from different spots in Fig. 2a4

3.2 Electrical properties

Figure 4 shows the Arrhenius plot of the resistivity of the composites in the temperature ranging from −35 to 50 °C. It can be seen that the composites exhibit typical NTC effect. Thermal constant B and ρ _25 °C of (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 as a function of doping CaO content are presented in Fig. 5. The resistivity of the composites first decreases and then increases as x value increases from 0 to 0.3. When x = 0.3, the value of resistivity is even larger than the CaO-free samples. In contrast, the thermal constant B increases sharply from 1,639 to 2,962 K with x increasing from 0 to 0.3. Therefore, a small quantity (≤0.2) of CaO-doped can produce a composite which possessed a right combination of resistivity and thermal constant (ρ _25 °C ≤ 8.61 Ω cm, B ≥ 2,690 K) required for the application of suppressing the inrush current. The aging coefficient (ΔR/R ₀) for the composites is shown in Fig. 6. It can be seen that the values of ΔR/R ₀ decreases with an increase in x. For x = 0.3, the resistivity of the composite was increased by 0.34% after aging at 125 °C in air for 500 h, while 5.02% for the samples without doping CaO. It is likely that the presence of CaO provides a physical barrier to the aging of the composite [15].

Fig. 4

Arrhenius plot of resistivity for (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50

Fig. 5

Thermal constant B and ρ _25 °C of (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 as a function of mole fraction of the doped CaO x

Fig. 6

Aging coefficient ΔR/R ₀ versus x for the (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 composites

As to the electrical conduction of the composite ceramics, it can be explained well by the ions migration mechanism and the ions migration mainly happens at B sites between the spinel structure NiMn₂O₄ and perovskite structure La_1−x Ca _x MnO₃ as the followed Eq. 1.

$$ {\text{La}}_{ 1- x} {\text{Ca}}_{x} {\text{MnO}}_{ 3} + {\text{Ni}}_{ 1- a} {\text{Mn}}_{a} \left[ {{\text{Ni}}_{a} {\text{Mn}}_{ 2- a} } \right]{\text{O}}_{ 4} \to {\text{La}}_{ 1- x} {\text{Ca}}_{x} \left[ {{\text{Mn}}_{ 1- b} {\text{Ni}}_{b} } \right]{\text{O}}_{ 3} + {\text{ Ni}}_{ 1- a} {\text{Mn}}_{a} \left[ {{\text{Ni}}_{a - b} {\text{Mn}}_{ 2- a + b} } \right]{\text{O}}_{ 4} $$

(1)

The electrical conduction of the composite is mainly determined by the amount of the carriers and their mobility, especially for manganite, the ion pairs of different valence states, Mn³⁺/Mn⁴⁺, for example, influence the electronic transport behavior markedly [16]. CaO-doped changes the ratio of Mn³⁺/Mn⁴⁺ in LaMnO₃ which effects the ion migration in the composite ceramics. The relationship between resistivity and x value in the composite ceramics can be explained as follows. For undoped LaMnO₃ where all the Mn ions are nominally in the tervalence state, predominant carriers are holes at room temperature. When La³⁺ is substituted by a small amount of Ca²⁺, some of the Mn³⁺ ions shift their valence state from tervalence to tetravalence, which contributes to the enhancement of hole concentration. Consequently, the drop in the resistivity of x = 0.1 composite sample is considered to be associated with the increase of the hole concentration in the perovskite phase. Due to the radius of Ca²⁺ is a little smaller than La³⁺, the increase of x induces the enhancement of lattice distortion and further decreases of interplanar spacing, which bates the Ni²⁺ ions migration between two phases because it also reduces the interplanar spacing of the perovskite structure La_1−x Ca _x MnO₃ with x = 0.2, 0.3. Therefore, the ion pair number of different valence states (Mn²⁺/Mn³⁺, Mn³⁺/Mn⁴⁺) decrease, then the electrical conduction of the composite weakens. Therefore, as the amount of CaO increases, the resistivity of the composite ceramics decreases to a minimum value and then increases, which is shown in Fig. 4.

The change of the composite electrical conductivity above can be confirmed further by the XPS results. Figure 7 shows the XPS spectra for the Mn 2p level of the samples NiMn₂O₄, La_0.9Ca_0.1MnO₃ and (NiMn₂O₄)_0.50(La_0.9Ca_0.1MnO₃)_0.50. In Table 3 it is summarized the binding energy and peak intensity value obtained from the XPS experiments using the approximate relationship: $\frac{{n_{1} }}{{n_{2} }} = \frac{{I_{1} }}{{I_{2} }} \cdot \frac{{\sigma_{1} }}{{\sigma_{2} }}\left[ {\frac{{Ek_{2} }}{{Ek_{1} }}} \right]^{\frac{1}{2}}$where n ₁, Ek ₁, and σ ₁ are the number of atoms of element I, the kinetic energy value corresponding to the line considered, and the cross-section calculated theoretically, respectively[17]. From Fig. 6 and Table 3, it can be seen that the valence states of the manganese ions have a highly mixed state of Mn²⁺, Mn³⁺ and Mn⁴⁺ at B site. Before compositing, the substitution of Ca²⁺ for La³⁺ in LaMnO₃ leads to the Mn³⁺ ions translating into Mn⁴⁺, which produces more ion pair Mn³⁺/Mn⁴⁺ and contribution to the high conductivity of the perovskite structure [16]. In the composite, in order to keep charge conservation, the migration of Ni²⁺ ions between two phases results in Mn⁴⁺ ions translating into Mn³⁺ in the spinel phase and Mn³⁺ ions translating into Mn⁴⁺ in the perovskite phase, and the latter is predominant which is in good agreement with the previously analysis and the peak intensity data in Table 3.

Fig. 7

The Mn 2p spectra for NiMn₂O₄ (a), La_0.9Ca_0.1MnO₃ (b) and (NiMn₂O₄)_0.50(La_0.9Ca_0.1MnO₃)_0.50(c)

Table 3 The binding energy and paek intensity of Mn 2p_3/2 obtained from the XPS spectrum

4 Conclusion

The composite ceramics (NiMn₂O₄)_0.50(La_1−x Ca _x MnO₃)_0.50 (x = 0, 0.1, 0.2, 0.3) consisted of spinel structure NiMn₂O₄ and perovskite structure La_1−x Ca _x MnO₃ were prepared by the conventional solid state reaction. Introduction of low-resistivity perovskite CaO-doped LaMnO₃ into the spinel oxide NiMn₂O₄ results in a decrease of the grain size from 6.5 to 2.0 μm, a large increase in B from 1,639 to 2,962 K as x increases from 0 to 0.3. The resistivity firstly decreases slightly with increasing CaO content from x = 0 to 0.1, and then in the range of x ≥ 0.1, the resistivity increases as the amount of Ca content increases, however, the ρ _25 °C is always <8.61 Ω cm. Doping CaO in LaMnO₃ make the composite NTC thermistors have better electrical stability (ΔR/R ₀ = 0.34% when x = 0.3) in comparison with CaO-free thermistors (ΔR/R ₀ = 5.02% when x = 0). The electrical conduction of the two phase composite can be well described using the ion migration mechanism.